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Our work involves detection of transient radicals species via flash photolysis type experiment. We incorporate a ultra-low-noise, quasi-CW (picosecond pulse), tunable Ti:sapphire laser to probe combustion relevant radicals to help improve predictions by RMG,

The spectral range of the laser, when used with a harmonic generator, covers most of the visible wavelengths allowing for the detection of a wide array of organic radical species. Paired with a temperature and pressure controlled slow flow reactor equipped with a Herriott-type optical multiple pass cell, transient absorptions of ~0.0001 can be measured, corresponding to cross section - concentration products of less than 1x10^-7 cm^-1. The flexibility and high sensitivity of this instrument allows direct and accurate measurement of many important transient intermediates in combustion and atmospheric chemistry.

Research

Experimental Setup

– Ti:sapphire laser pumped with CW - 532nm

– 230 - 540 nm light is obtained by doubling/tripling the fundamental frequency.

– 230 nm to 1.2 µm light is obtained using optical parametric oscillator (OPO) and doubling/tripling optics.

– Reference beam, differential amplification used to cancel laser noise.

– Io is continuously sampled using a beam splitter and optical chopper.

– Differential amplified used to reduce the laser noise further.

– Absorption is measured in a temperature and pressure controlled slow flow reactor. T = 298-700K, P = 0-1 atm.

– Herriott mirrors coated for >99% reflection of 230-540 nm. Up to 55 passes (up to 40 m total absorption path length).

– Pressure controlled by an automatic butterfly valve via Labview

– Gas flow rates regulated by mass flow controllers operated via Labview.

– VI calculates concentrations, retrieves, processes, and saves oscilloscope data.

– Temperature controlled by three independent PID loops.

– Absorption data retrieved from oscilloscope, normalized, and converted to Igor text file in Labview.

– Instantaneous pressure and temperature control allows for data collection at a rate of 3 temperatures per day.

– Modular design of reaction chamber allows for quick, easy disassembly without alteration of Herriott mirror alignment or thermocouple position.

– Equipment modifications to study new chemical systems can be completed in less than 1/2 day.

Computer Automation

Chemical Systems

- Self Reaction of Vinyl (C₂H₃) radical

Vinyl radicals (C₂H₃) are important intermediates in combustion and atmospheric chemistry. Relatively few experimental studies have been reported despite its importance. This is primarily due to the difficulty in generating and detecting a clean source of vinyl radicals. Therefore, Vinyl Iodide, is chosen as precursor, which is a clean source of vinyl radicals.

The fourth harmonic of a Nd:YAG (266nm) is used a photolysis laser. Vinyl radical is probe at several different lines to get its temporal profiles. The initial concentration of vinyl molecules produced by each photolysis event was determined using direct laser absorption by the I atom at 1315 nm. The time-resolved vinyl radical concentration is fit to a second order kinetic equation to extract the self-reaction rate constant.

This project is part of an on going collaboration with Prof. Askar Fahr from Howard University and Dr. Craig Taatjes from Sandia National Laboratories. Prof. Fahr has measure the vinyl self reaction rate to be 1.21 x 10^-10 (cm³/molecules s). Where as Dr. Taatjes measured the self reaction rate to be 4.0 x 10^-11 (cm³/molecules s). The goal for ours experiments is resolve this discrepancy.

Our present study agrees with Sandia National Laboratory. We believe that the vinyl self reaction rate to be lower by a factor of two from previous studies.

- Vinyl (C₂H₃) reaction to alkenes

Vinyl Radicals (C₂H₃) are highly reactive, capable of adding to unsaturated hydrocarbons. This addition process leads to the formation of polycyclic aromatic hydrocarbons, soot. Understanding the processes that lead to soot formation is a key step in their reduction.

The addition of vinyl plus alkene (C₂H₄) is outlined as shown below. k₁ represents the major pathway of the reaction in the high pressure limit, but the reaction can proceed along K₄ and K₅ when it is thermally activated or in the low pressure limit. The combined rate, vinyl + ethene a products has been studied by Shestov et. al. at low pressure and T = 625-950K, but values for the individual rate constants have not been determined.

Employing pseudo-first order kinetic methods, we have measured rate constants for the reactions of vinyl with several alkenes, including: ethene, propene, isobutene, 1-butene, and 2-butene.

Time decay of C₂H₃ at 550 K and 15 torr and [Vinyl Iodide]=1e15 molecules/cm³

Pseudo-first-order C₂H₃ decay rate k′ vs. [C₂H₄]

The current study found the Arrhenius parameters to be A = 1.34x10^-12 cm³ molecule^-1 s^-1 and E_a = 19.7 kJ mol^-1 K^-1 for vinyl + ethene.. These values are in agreement, within the error bar, with Shestov. These result also match very well with theory (B3LYP) for the case of Ethene. For all the other alkenes systems, higher level theory is require for better E_a prediction. Also, we need implement hindered rotors to improve A-factor prediction.